Whoa. But also: aside of IPs, the log should have visited URLs and referrers, right? This would give (or at least strongly hint at) answers to most of the obvious questions. My bets are, either A. spambots from some site crawled by a botnet (which failed to create accounts?), or B. Winterpox raised a swarm of attack tumblrinas one link popped up in some "big" feed somewhere on social networks.
People never wonder How the world goes round -Helloween And even I make no pretense Of having more than common sense -R.W.Wood It's not good, Eric. It's a gazebo. -Ed Whitchurch
I'm assuming that a link popped up somewhere and there was a lot of bots hitting us for feed content, or there was some sort of search engine glitch somewhere. I don't have access to the info that Big Al does, but having nothing more than a high number of visitors speaks against it being anything malicious.